Combination valve assembly with proportioner override

ABSTRACT

A combination valve assembly for a dual circuit brake system in which a metering valve is in one circuit, a proportioning valve is in the other circuit, and a hydraulically reset pressure loss warning indicator unit is sensitive to both circuits. Actuation of the warning unit because of pressure loss in the metering valve circuit causes the proportioning valve to be overriden by removing one of the valve elements from a proportioning position so that pressure through the valve is not proportioned.

United States Patent 1 Rike et al.

[54] COMBINATION VALVE ASSEMBLY WITH PROPORTIONER OVERRIDE [75]Inventors: Richard C. Rike, Dayton; Ronald L.

Shellhause, Vandalia, both of Ohio [7 3] Assignee: General MotorsCorporation,

Detroit, Mich.

[22] Filed: Sept. 17, 1970 [21] Appl. No.: 72,992

[52] US. Cl. ..303/6 C, 251/368 [51] Int. Cl. [58] Field of Search..303/6 C, 84 A; 200/82 D; 340/52 C; 188/151 A; 251/368 [56] ReferencesCited UNITED STATES PATENTS 3,441,318 4/1969 Bueler ..303/6 C 3,464,7419/1969 Falk ...303/6 C 3,556,607 1/1971 Shutt et al ..303/6 C INDICATOR1 May 15, 1973 2,904,877 9/1959 Edelen ..25l/368 X 2,981,514 4/1961Loxham ..25l/368 X 3,480,333 11/1969 Stelzer ..303/6 C 3,572,850 3/1971Schultz .303/84 A 3,498,681 3/1970 Bueler ..303/6 C PrimaryExaminer-Evon C. Blunk Assistant Examiner-Merle F. Maffei Attorney-W. E.Finken and D. D. McGraw [57] ABSTRACT A combination valve assembly for adual circuit brake system in which a metering valve is in one circuit, aproportioning valve is in the other circuit, and a hydraulically resetpressure loss warning indicator unit is sensitive to both circuits.Actuation of the warning unit because of pressure loss in the meteringvalve circuit causes the proportioning valve to be overriden by removingone of the valve elements from a proportioning position so that pressurethrough the valve is not proportioned.

9 Claims, 1 Drawing Figure 211' Zfl 1% 2 282;

A 222 8) zze 28 a PATENTED HAY] 51915 m U s. Wmw. m mm W M Y B SUMMARYOF THE INVENTION The invention relates to a vehicle dual circuitcombination valve assembly, and more particularly to one having apressure loss warning unit and a proportioning valve unit withproportioner override. The override action occurs when pressure lossoccurs in the brake circuit not containing the proportioning valve, andin an uncorking type of action in which a normally fixed valve elementis moved away and out of proportioning operating range of the movableproportioning valve element to permit direct and unproportional deliveryof brake pressure through the proportioning valve. This provides asingle, direct acting arrangement in which no by-pass mechanism andpassages are required. Pressure continues to follow the same path asbefore instead of being rerouted through a by-pass. One of the featuresof the invention is the provision of a lostmotion mechanism as a part ofthe override arrangement, resulting in better action of the warning unitand the proportioner. Another feature of the invention is the pilotingarrangement for one of the proportioner valve elements which reducesconcentricity tolerance requirements while allowing free, non-bindingstroking of the valve element. This includes two aspects, one being aswiveLpilot construction, and the other being a soft metallic coatingwhich conforms quickly to the actual alignment of parts to provideconcentricity. Another feature of the invention is the combination ofthe warning and proportioning and proportioning override features with ametering valve in the other circuit.

IN THE DRAWING The single FIGURE illustrates mechanism embodying theinvention, with parts in section, and with a schematic showing of thebrake system and warning circuits.

DESCRIPTION OF THE PREFERRED EMBODIMENT The housing is provided with athrough bore 12 having several shoulders thereon and divided intoseveral chambers as will be describedv The front brake pressure inlet 14connects with a left center portion of the bore 12, and outlets l6 andI8 connect with the left end portion of the bore 12. These outlets areconnected to the two front disc brakes and 22 of the vehicle. The rearbrake pressure from the master cylinder 24 passes through inlet 26,connected to the right center portion of the bore 12, and the outlet 28at the right end of the bore 12 is connected to the rear brakes 30 and32.

The left portion 34 of the assembly comprises the metering section,which prevents initial front disc brake pressure buildup until the rearbrake shoes contact the drums. The pressure thus held off is thenadmitted to the front brakes at higher input pressures.

The center portion 36 of the assembly comprises a warning sectionincluding a latch-type hydraulic reset warning switch assembly andactuator. The switch assembly 38 is threaded into a housing opening 40so that the switch plunger 42 extends into the center portion of thehousing bore 12.

The right portion 44 of the assembly includes a proportioner sectioncomprising a rear brake pressure proportioner and a proportioneroverride arrangement.

METERING SECTION Front brake pressure enters inlet 14 and chamber 46,provided in a part of bore 12. The left side of chamber 46 is defined bythe vale pin stop 48. This stop is cup shaped, and is provided with oneor more openings 50 through which chamber 46 is connected to the chamber52 inside the cup. The metering valve pin 54 has a smooth head 56positioned in chamber 52 and separated by a groove 58 from a knurled pinsection 60. The pin extends leftwardly to a reduced pin section 62 aboutwhich diaphragm seal 64 is mounted. The pin land 66 is slidably mountedin a bore 68 formed through the valve plug 70. The pin reduced end 72extends outwardly through the valve plug and is covered by the meteringvalve boot 74. The valve plug is held in the left end of the housingbore by retainer ring 76.

The chamber 78 between diaphragm seal 64 and the valve pin stop 48contains the metering valve 80. The outer portion 82 of valve acts as avalve which sealingly engages a valve seat 84 formed by the valve pinstop 48 and a shoulder 86 of bore 12. The annular portion of meteringvalve 80 has an axially extending annular section or lip 88 which (inthe position shown) receives and engages the outer periphery of the pinknurled land 60. A valve retainer and spring seat 90 presses against theother side of metering valve 80 relative to valve pin stop 48 and isengaged by compression spring 92, which also acts against spring seat 94and the outer periphery of diaphragm seal 64. The portion of pin 54intermediate land 60 and section 62 has a groove 96 receiving a springseat 98. Spring seats against spring seats 94 and 98 and urges pin 54rightwardly toward engagement by the end of head 56 with pin stop 48.The front brake pressure outlets l6 and 18 connect with chamber 78.

When brake pressure is initially applied, fluid passes through inlet 14,chamber 46, passages 50, chamber 52, and the axially grooved openingsformed by the knurled land 60, to chamber 78. This initial pressure isbetween 0 and 30 psi. This light initial pressure in chamber 78 istransmitted to the front brakes 20 and 22 and also acts on diaphragmseal 64 to move metering valve pin 54 to the left against spring 100until the shoulder 102 of the knurled land 60 engages the inner annulus104 of spring retainer 90. This provides a first stop for the pin 54, atwhich time the smooth outer periphery of pin head 56 has moved to engagethe lip 88 of the metering valve 80, blocking off further flow of fluidfrom chamber 52 to chamber 78.

Additional inlet pressure must now be built up before additionalpressure can be supplied through the valve assembly to the front brakes.This pressure is called the hold-off" pressure and is controlled by theeffective area of the metering valve 80 and the load of spring 92. Thehold-off" pressure may be varied from 60 to 200 psi as required for aparticular vehicle.

Continued increase in inlet pressure in chambers 46 and 52 acts on theeffective area of the metering valve 80 and the head 56 of the meteringvalve pin 54 and eventually causes the pin 54, the valve 80, and thespring retainer 90 to move leftwardly against the forces of springs 92and 100 to open the valve 82 relative to its seat 84. Inlet pressure isthen admitted to chamber 78 past the outer periphery of valve seal 80and acts on diaphragm seal 64 to move pin 54, with retainer 90 and valve80, further to the left until the pin land 66 engages the shoulder l06at the left end of bore 68. A transition between the inlet pressure andthe pressure to the front brakes takes place as this occurs and thistransition is completed at a pressure point called the blend pressure.The inlet pressure is thereafter the same as the pressure passingthrough outlets l6 and 18 to the front brakes. The blend pressure iscontrolled by the relationship of the effective area of diaphragm seal64 and the spring load of springs 92 and 100.

Upon release of inlet pressure, pin 54 gradually moves rightwardly as dovalve spring retainer 90 and valve 80, until the metering valve 80 againhas its outer periphery 82 seating against the valve seat 84. As furtherrelease of inlet pressure occurs, the lip 88 of the metering valve liftsto allow pressure to flow from chamber 78 to chamber 52. This allowsrelease of pressure to the front brakes 20 and 22 at a very smallpressure differential. Upon complete release, pin 54 assumes theposition shown so that chamber 78 is again connected to chamber 52through the grooves formed by the knurls of pin land 60. These knurlsand grooves provide free flow of brake fluid for compensation of changein volume of the hydraulic circuit due to thermal changes.

WARNING SECTION The switch assembly 38 has a housing 108 threaded intoan appropriate opening 40 in housing 10. A terminal 110 extendsoutwardly and is separated from housing 108 by insulator 112 and O-ring114. Terminal 110 is electrically connected through a suitable switch116 to an indicator 118 and a source of electrical energy schematicallyillustrated as battery 120. Indicator 118 may be a light, bell, or horn,by way of example. The inner end ofterminal 110 is formed as a pin 122around which steel collar 124 is received. Collar 124 extends downwardlybeyond the end of pin 122. The upper end of plunger 42 is formed as asomewhat similar but slightly larger diameter pin 126 which is normallyseparated from pin 122 and is movable upwardly to engage collar 124 toclose the switch contacts. A spring 128 urges terminal 110 with itscollar 124 upwardly by acting through an insulated spring seat 130 whileurging plunger 42 downwardly. A plastic insulator 132 about a reducedportion of the plunger pin 126 guides pin 126 in collar 124 andinsulates them when the switch is open. Plunger 42 is grounded throughassembly housing as schematically illustrated by ground wire 134. Thelower end of plunger 42 extends into the center portion of the bore 12formed in housing 10.

The switch piston assembly 136, including piston 138, is positioned inthe center portion of the bore 12 of housing 10 so that the pistongrooved center section 140 is normally aligned with plunger 42, theplunger end being held in groove 140 by the force of spring 128. Thegroove has beveled sides 142 and 144 leading to shoulders 146, 148 andso positioned that movement of piston assembly 136 to either aleftwardly or rightwardly direction causes plunger 42 to be cammedupwardly to close contacts 124 and 126. The contacts are held in theclosed position by engagement of the lower end of plunger 42 on one ofthe shoulders. The slightly larger lands 150 and 152 of shoulders 146and 148 aid in holding plunger 42 on a shoulder once it has beenpositioned there.

The right end of piston 138 is formed as an enlarged land 154 providedwith a seal 156. Land 154 and sea] 156 guide and seal piston 138 in theright center position of bore 12. The left end of piston 138 has a land158 of the same diameter as land 154 and guides piston 138 in the leftcenter portion of bore 12. The left outer end 160 of the piston assemblyis formed as a land 162 of smaller diameter than lands 154 and 158. Aseal retainer 164 slidably fits over land 162 and has a smaller innerdiameter than the diameter of lands 154 and 158, but a larger outerdiameter than the diameter of those lands. For this purpose, the housingbore 12 is provided with a shoulder 166 against which retainer 164normally is abutted. An O-ring seal 168 is received about the land 158intermediate the retainer 164 and another sleeve-like retainer 170.Retainer 170 has the same inner and outer diameters as retainer 164.Retainers 164 and 170 and the seal 168 may move relative to piston 138under certain conditions.

The left end 160 of piston 138 and one side of retainer 170 are exposedto master cylinder front brake pressure in chamber 46. The right end 154of piston 138 is exposed to master cylinder rear brake pressure inchamber 172. Rear brake pressure inlet 26 is connected to the mastercylinder 24 and to chamber 172.

The piston assembly 136 also has a recess or bore 174 formed in theright end of piston 138 and opening into chamber 172. Spring 176 fits inthe inner end of bore 174 and seats on a guide 178 which is slidablyreceived in bore 174. Guide 178 may be in the form of a grooved land, ormay be a fluted member. In either case, it is secured to or formed as apart of a valve element of the proportioner section 44 described belowin detail. An inwardly lanced tab 180 adjacent the open end of bore 174provides a stop for guide 178 such that sufficient leftward movement ofpiston 138 will cause tab 180 to engage guide 178 and thereafter movethe guide leftwardly with further leftward movement of piston 138. Thisprovides a lost-motion connection discussed below.

So long as substantially normal front and rear brake pressures exist inchambers 46 and 172, the switch piston assembly 136 will be in theposition shown. Should front brake pressure loss occur while rear brakepressure remains, the higher pressure in chamber 172 acting on the rightend of piston 138 will move the piston assembly 136 leftwardly. Land 158will cause seal retainers 164 and 170, as well as sea] 168, to also moveleftwardly with piston 138. This movement will move plunger 42 upwardlyand over land 152 until it rests on shoulder 148. This will hold switchcontacts 124 and 126 engaged, holding the warning circuit in theenergized condition. When, for example, switch 116 is the vehicleignition switch, indicator 118 will be energized so long as the ignitionswitch is closed and piston 138 remains in the actuated condition.

The warning section is hydraulically reset when the front brake systemis repaired and front brake pressure is again applied in chamber 46.This pressure will act against the larger combined effective areas ofseal retainer 170 and the left end 160 of piston 138 (as compared to theeffective area of the right land of piston 138) to move piston assembly136 rightwardly until the seal retainer 164 again engages the shoulder166. This recenters the piston assembly and disengages the warningswitch contacts 124 and 126.

Should rear brake pressure fall substantially below front brakepressure, the piston 138 moves rightwardly, with plunger 42 movingupwardly over land 150 and resting on shoulder 146. When the rear rakepressure is again available in chamber 172, the larger effective area ofthe right end of piston 138 relative to the effective area of the pistonleft end 160 permits rear brake pressure to move the piston 133 to theleft until its land 158 engages retainer 164. Thus, the position shownis again assumed.

PROPORTIONER SECTION The right end of the bore 12 through housing isformed to provide chamber 172 and a larger chamber 182. The shoulder 184separating the two chambers receives the annular flanged seal supportmember 186. The proportioning valve cap member 188 is threaded intochamber 182 and abuts against seal support 186 to hold it in place.Valve cap 188 has outlet 28 formed axially therein and has a chamber 190in its inner end to which the outlet 28 is connected. The proportioningvalve piston 192 is reciprocally received in chamber 190 and extendsinto chamber 172. lt'passes through seal support 186, as well as theV-block seal 194. Seal 194 faces chamber 172 andis supported by sealsupport 186.

The center flange 196 of piston 192 is piloted on the chamber wall 198of chamber190. Thus, chamber wall 198 is a bore and piloting surface.The reduced diameter outer end 200 of piston 192 provides a mounting forV-block seal 202, which faces outwardlytoward chamber 190 and is backedup by flange 196. A piston stop 204 is mounted in the end of chamberl90adjacent outlet 28. Piston stop 204 has one or more openings orradial slots 206 which provide for fluid connection between chamber 190and outlet 28. The inner end 208 of piston 192 receives spring retainer210 to provide a seat for compression spring 212 and springguide 214.Thus, in the assembled position, spring 212 urges piston 192 to theright so that it abuts piston stop 204.

The piston stop 204 is formed to provide openings 206 of sufficient areaso that the piston through-bore 216 is always fluid connected withchamber 190 and outlet 28 in unrestricted flow relation. The end of bore216 is formed at inner piston end 208to provide a valve seat 218. Valveseat 218 is one valve element of the proportioning valve, and is theelement that moves during the usual proportioning action. The valve stem220 extends through piston 192 so that its outer end 222 normally abutspiston stop 204. A center portion of valve stem 220 is provided with avalve element 224 which mates with valve seat 218. The part of valvestem 220 within bore 216 is sufficiently smaller than the bore to permitrelatively unrestricted flow therethrough between chambers 172 and 190,subject to the valving action of valve element 224 and valve seat 218.The inner end 226 of valve stem 220 extends into piston bore 174 and hasguide 178 formed thereon or secured thereto by suitable means. Thus, thelost motion connection is provided between valve stem 220 and the switchpiston 138.

The mechanism is in the position illustrated before brake pressure isapplied. Upon application of master cylinder rear brake pressure throughinlet 26 and into chamber 172, pressurized fluid passes between valveelement 224 and valve seat 218, through bore 216 and openings 206, andinto chamber 190. It also passes through openings 206 into outlet 28. Asthe pressure builds up, it acts on the piston differential area definedby the area of piston 192 exposed to chamber relative to the piston areaexposed to chamber 172 so as to move piston 192 to the left against theforce of spring 212. Since pressure is being built up substantiallyequally and normally in the front and rear brake supply lines, thehydraulic forces do not move the switch piston 138. As the rear brakepressure continues to build up, piston valves seat 218 moves towardvalve element 224. This causes a reduction in output pressure to therear brakes equivalent to the ratio of the differential areas of piston192. The valve element 224 is axially positioned for this proportioningfunction by its stem. During brake release the master cylinder acts todecrease the pressure in inlet 26 and chamber 172. As this pressuredecreases, the brake apply pressure in outlet 28and chamber 190 acts tomove the proportioning valve piston 192 leftwardly against spring 212 asseen in the drawing, until the center flange 196 engages the sealsupport and piston stop member 186. This results in an initial decreasein the pressure in chamber 190 due to chamber expansion as a result ofpiston displacement. Momentarily, during the decrease in supplypressure, the valve seat 218 functions in relation to normally fixedvalve 224 to reverse-proportion the pressure. However, due to the smallamount of movement available to piston 192, valve seat 218 maintainsengagement with valve 224 as the piston moves leftwardly, moving thevalve 224 with it against the relatively light force of lost-motionspring 176. When the brake supply pressure in chamber 172 has decreasedsufficiently, valve 224 will open relative to valve seat 218, furtherrelieving the brake apply pressure in chamber 190. As the pressure inchamber 190 decreases due to fluid flow through the open valve, theforce exerted on piston 192 by the pressure in this chamber becomesinsufficient to oppose the force of spring 212, which was furthercompressed by the leftward movement of piston 192. The unbalanced forcesthen acting on piston 192 cause it to move sharply to the right until itengages piston stop 204. Since this sharp movement would otherwiseresult in an audible click-type noise, which may be objectionable, arubberlike snubber 227 is provided on the right end of piston 192 sothat the snubber engages piston stop 204 prior to metal-to-metal contactbeing made by the end of the piston against the piston stop. The snubberis then compressed by the force of spring 212 so that metal-tometalcontact may be attained. This prevents the full compressive loading ofthe spring 212 from being taken by the snubber at all times when piston192 is positioned against the piston stop 204. Since the force of spring176 is light, and the lost-motion mechanism is provided, valve 224 andits stem 220 can move for some distance toward switch piston assembly136 during the brake release action above described without exerting anysignificant force thereon. When valve 224 opens relative to seat 218during brake release, under influence of the relatively high pressure inchamber 190, the proportioning valve operates to permit a quick andrelatively unrestricted return of brake fluid to the master cylinder. Asshown in the drawing, when the proportioning valve is at rest, valve 224and valve seat 218 are not engaged, therefore maintaining fluidcommunication between chambers 172 and 190.

Should the front brake pressure fall substantially below the rear brakepressure, additional work must be done by the rear brakes, and thisrequires additional rear brake pressure. To get the maximum brakingeffort under this condition and yet keep brake pedal effort as low aspossible, the action of the proportioner is overridden and held out ofthe system. Thus, when switch piston 138 moves leftwardly due to frontbrake pressure loss, the guide 178 is engaged by the tab 180 of piston138 and pulls the valve stem 220 leftwardly to open valve element 224well away from its seat 218. This forcible removal of the valve element224 from its proportioning position allows full rear brake pressure topass from chamber 172 through bore 216 to outlet 28. The leftwardmovement of valve piston 192 is limited by the engagement of the pistonflange 196 with the outer side of seal support member 186. Thus, valveseat 218 cannot move leftwardly a sufficient distance to reinstate theproportioning action. By connecting the normally fixed valve element224, through its valve stem 220 and the lost motion connection, to theswitch piston 138, so that it functions as a part of the overridemechanism instead of so connecting the normally movable valve element218, the movable valve element is allowed to function in its normalproportioning mode so that the normal proportioning action is completelyunaffected by the presence of the lost motion connection.

A vent groove 228 allows sufficient expansion of chamber 182 pastV-block seal 230 for proper proportioner function. Should either seal194 or 202 leak brake fluid to chamber 182, collection of fluid willtake place in groove 228. However, seal 230 will permit the release ofexcess fluid leakage collection to allow proper proportioner function.The seal 230 can also function as a vacuum bleeding seal, allowingvacuum bleeding of new cars on assembly lines.

In the proportioning section 44 it is an important feature of theinvention that the proportioning valve piston 192 be aligned for freestroking movement in the valve cap member 188 as well as in the sealsupport member 186. The bore and pilot surface 198 of the valve capmember 188 is engaged in a sliding and piloting relationship by theouter peripheral surface of the center flange 196 of piston 192. Thepiloting surface 232 formed by the inner peripheral surface of sealsupport member 186 is also slidably engaged in a piloting relation withpiston 192. In turn, the piloting surface 234 of seal support member 186engages the cylinder wall portion of bore 12 forming chamber 172 andadjacent shoulder 184.

In order for all of the piloting surfaces to be correctly related, aclose tolerance relationship is also required for the threaded areabetween valve cap member 188 and the threaded portion of bore 12 formedin housing 10. In the usual construction, this would therefore re quirethe close coordination of four piloting surfaces which would be soaligned as to prevent any binding of piston 192 in its axial movement.in order to alleviate the extremely close tolerance required for such anarrangement, the seal support member 186 has a short bearing lengthforming the piloting surface 232. This is provided by the undercut 236.

The axially short length of piloting surface 232 allows for a slightswiveling action to obtain alignment relative to that piloting surfacewhich would not be obtainable should the seal support member have anaxially extending piloting surface which would prevent any such minuteswiveling action. Furthermore, a soft metallic plating provided on thepiloting surface 232 prior to assembly permits a further alignment ofthe final piloting surface resulting in a better free stroking action.During installation of the valve piston 192, the soft metallic platingdeforms and quickly wears away in a stroke or two so that the finalalignment of piloting surface 232 conforms more specifically to thealignment of the piston center flange 196 on the piloting surface 198.It has been determined that the soft metallic plating should have aminimum thickness of 0.0002 inch and a maximum thickness of 0.0004 inch.Plating thicknesses less than the minimum are insufficient to compensatefor a misalignment occurring within the desired tolerance level, andthose greater than the maximum thickness are found to wear away furtherand result in an overly loose piloting fitting, creating problems of anoverly loose valve piston 192. In the preferred embodiment, the platingis made of tin. However, it can also be of other suitable soft metallicmaterial, such as lead or cadmium. It must have the soft characteristicso that it allows conformation of alignment quickly and accurately withthe installation stroke of piston 192 or only a stroke or twothereafter.

What is claimed is: 1. In a brake system for a vehicle having first andsecond separate brake sub-systems:

brake system pressure loss indicating means for providing a signal inresponse to pressure loss in either of said brake sub-systems, saidindicating means comprising pressure responsive means having first andsecond opposed effective areas respectively responsive to the pressuresin said first and second brake sub-systems, and normally being in afirst condition in response to the pressures in both subsystems beingapproximately normal, and being actuatable to a second condition inresponse to pressure loss in either but only one sub-system below theapproximately normal sub-system pressures, and signal means forproviding said signal in response to said pressure responsive meansbeing actuated to said second condition; proportioning valve means inseries with one of said first and second brake sub-systems with theinput to said proportioning valve means being in fluid pressurecommunication with one of said first and second opposed effective areas,said proportioning valve means having a normally fixed valve elementprovided as a part of a vale stem and an annular valve element receivingsaid valve stem therethrough in radially spaced relation to define anannular flow path and movable relative to said normally fixed valveelement to proportion brake apply pressure through said annular valveelement in said one brake sub-system in relation to pressure supplied tothe input to said proportioning valve means through said one brakesub-system; and override means actuatable to override said proportioningvalve means and including said normally fixed valve element and valvestem of said proportioning valve means and lost motion means, formed ofa recessed end of said pressure responsive means and a compressionspring and a head on said valve stem in said recessed end and stop meansadjacent the opposite end of said annular valve element and normallyengaged by the end of said valve stem to define the normally fixedposition of said normally fixed valve element, and being actuatable tomove said valve stem out of engagement with said stop means and saidnormally fixed valve element beyond the range of movement of saidannular valve element in response to actuation of said pressureresponsive means to said second condition in response to pressure lossonly in the other of said first and second brake sub-systems, andlatching means so holding said pressure responsive means and said valvestem and said normally fixed valve element and subject to normal brakeactuating pressures in said first and second sub-systems to release saidpressure responsive means for return thereof to said first condition.

2. The mechanism of claim 1, said override means lost motion meansincluding linkage connecting said pressure responsive means and saidproportioning valve means normally fixed valve element and acting toallow a predetermined initial amount of movement of said pressureresponsive means while permitting proportioning of pressure when thepressures in said first and second brake sub-systems are approximatelynormal,

and acting only after said predetermined initial amount of movement ofsaid pressure responsive means to an actuating position in response topressure loss in the other of said first and second brake sub-systems toremove said normally fixed valve element to an open valve positionpermitting pressure in said one brake system to pass through saidproportioning valve means without being proportioned.

3. The mechanism of claim 1 further comprising metering valve means inseries with the other of said first and second brake sub-systems withthe input to said metering valve means being in fluid pressurecommunication with the other of said first and second piston effectiveareas.

4. In a brake system for a vehicle having first and second separatebrake sub-systems:

brake system pressure loss indicating means for providing a signal inresponse to pressure loss in either of said brake sub-systems, saidindicating means comprising pressure responsive means having first andsecond opposed effective areas respectively responsive to the pressuresin said first and second brake sub-systems, and normally being in afirst condition in response to the pressures in both subsystems beingapproximately normal, and being moved through a predetermined amount ofinitial movement and therebeyond to a second condition in response topressure loss in either but only one sub-system below the approximatelynormal sub-system pressures; and

signalmeans for providing said signal in response to said pressureresponsive means being actuated to said second condition;

proportioning valve means in series with said second brake sub-systemwith the input to said proportioning valve means being in fluid pressurecommunication with said second opposed effective area of said pressureresponsive means, said proportioning valve means having a first valveelement normally having a fixed position and a second valve elementnormally movable relative to said first valve element in response to thepressure in said second brake sub-system at the input to saidproportioning valve means to proportion pressure at the output of saidproportioning valve means; and proportioning valve means override meansincluding said first valve element and lost motion connecting meansreceived within and including one end of said pressure responsive meansand connecting said first valve element and said pressure responsivemeans for movement of said first valve element in a direction away fromsaid second valve element when there is a pressure loss only in saidfirst brake subsystem which is sufficient to move said pressureresponsive means beyond said predetermined amount of initial movement tooverride the proportioning action of said proportioning valve means andpermit full transmission of second brake subsystem pressure through saidproportioning valve means between said valve elements.

5. In a brake system for a vehicle having first and second separatebrake sub-systems:

brake system pressure loss indicating means for providing a signal inresponse to pressure loss in either of said brake sub-systems, saidindicating means comprising pressure responsive means having first andsecond opposed effective areas respectively responsive to the pressuresin said first and second brake sub-systems, and normally being in afirst condition in response to the pressures in both subsystems beingapproximately normal, and being actuatable to a second condition inresponse to pressure loss in either but only one sub-system below theapproximately normal sub-system pressures, and

signal means for providing said signal in response to said pressureresponsive means being actuated to said second condition;

and proportioning valve means in series with one of said first andsecond brake sub-systems with the input to said proportioning valvemeans being in fluid pressure communication with one of said first andsecond opposed effective areas, said proportioning valve means includinga housing having a bore therein, an annular seal support member and anannular valve cap member mounted in said housing bore, said members andsaid housing bore being substantially axially aligned and said membersreceiving therein a pressure responsive movable proportioning valveelement in reciprocably piloted relation,

said annular valve cap member having the piloting surface thereofextending axially a distance greater than the allowable movement of saidvalve element and said annular seal support member having an axiallyextending piloting surface of said valve element thereat for bind-freemovement of said valve element relative to the piloting surfaces of saidmembers.

6. The mechanism of claim 5, said other annular seal 65 support memberpiloting surface being coated prior to assembly with a soft metallicplating, of tin, lead or cadmium by way of example, to a radialthickness at least equal to the tolerance allowable for misalignment of1 1 said piloting surfaces, said plating being sufficiently soft to wearwith a few strokes of said valve element to establish a close alignmentand prevent valve binding.

7. The mechanism of claim 6, said soft metallic plating radial thicknessbeing at least 0.0002 inch.

8. The mechanism of claim 6, said soft metallic plating radial thicknessbeing in the range of 0.0002 inch to 0.0004 inch.

9. In a brake system for a vehicle having first and second separatehydraulic brake sub-systems:

a proportioning valve assembly in one of said brake sub-systems, saidvalve assembly having a brake supply pressure inlet and a brake applypressure outlet and a bore connecting said inlet and said outlet, aproportioning piston mounted in said bore for limited reciprocalmovement,

a first stop member mounted in said bore and limiting movement of saidpiston toward said inlet,

a second stop member mounted in said bore and limiting movement of saidpiston toward said outlet,

a first proportioning valve element formed on said piston,

a second proportioning valve element reciprocably mounted in said boreand having a normally fixed position defined by one of said stop membersand cooperating with said first proportioning valve element to controlfluid flow and pressure transmittal between said inlet and said outlet,first means yieldably urging said piston toward and into engagement withsaid second stop member,

second means yieldably urging said second proportioning valve elementtoward said first proportioning valve element, a snubbing member on oneend of said piston engaging said second stop member,

said piston having opposed differential areas with the smaller areabeing exposed to brake supply pressure at said inlet and the larger areabeing exposed to brake apply pressure at said outlet to generate a forceopposing said first yieldable means and at a predetermined brake supplypressure moving said piston and said first proportioning valve elementinto proportioning relation with said second proportioning valve elementto proportion the brake apply pressure relative to the brake supplypressure, the brake supply pressure decreasing upon brake release andthe brake apply pressure then acting on said piston against said firstyieldable means to move said piston and said proportioning valveelements toward said inlet and initially decrease said brake applypressure by piston displacement until said piston engages said firststop member, said second proportioning valve element being moved by thehigher brake apply pressure against the decreased brake supply pressureand the force of said second yieldable means to move away from saidfirst proportioning valve element to permit brake fluid flow from saidoutlet to said inlet and further decrease of brake apply pressure, saidfirst yieldable means then overcoming the force generated by thedecreasing brake apply pressure acting on said piston largerdifferential area and moving said piston away from said first stopmember toward said second stop member,

said snubbing member engaging said second stop member to cushion theengagement of said piston with said second stop member; and

said second yieldable means moving said second proportioning valveelement to its normally fixed position spaced from said firstproportioning valve element to maintain fluid communication between saidinlet and said outlet during the remainder of the brake release cycleand the initial part of the next brake apply cycle.

1. In a brake system for a vehicle having first and second separatebrake sub-systems: brake system pressure loss indicating means forproviding a signal in response to pressure loss in either of said brakesub-systems, said indicating means comprising pressure responsive meanshaving first and second opposed effective areas respectively responsiveto the pressures in said first and second brake sub-systems, andnormally being in a first condition in response to the pressures in bothsubsystems being approximately normal, and being actuatable to a secondcondition in response to pressure loss in either but only one sub-systembelow the approximately normal sub-system pressures, and signal meansfor providing said signal in response to said pressure responsive meansbeing actuated to said second condition; proportioning valve means inseries with one of said first and second brake sub-systems with theinput to said proportioning valve means being in fluid pressurecommunication with one of said first and second opposed effective areas,said proportioning valve means having a normally fixed valve elementprovided as a part of a vale stem and an annular valve element receivingsaid valve stem therethrough in radially spaced relation to define anannular flow path and movable relative to said normally fixed valveelement to proportion brake apply pressure through said annular valveelement in said one brakE sub-system in relation to pressure supplied tothe input to said proportioning valve means through said one brakesubsystem; and override means actuatable to override said proportioningvalve means and including said normally fixed valve element and valvestem of said proportioning valve means and lost motion means, formed ofa recessed end of said pressure responsive means and a compressionspring and a head on said valve stem in said recessed end and stop meansadjacent the opposite end of said annular valve element and normallyengaged by the end of said valve stem to define the normally fixedposition of said normally fixed valve element, and being actuatable tomove said valve stem out of engagement with said stop means and saidnormally fixed valve element beyond the range of movement of saidannular valve element in response to actuation of said pressureresponsive means to said second condition in response to pressure lossonly in the other of said first and second brake sub-systems, andlatching means so holding said pressure responsive means and said valvestem and said normally fixed valve element and subject to normal brakeactuating pressures in said first and second sub-systems to release saidpressure responsive means for return thereof to said first condition. 2.The mechanism of claim 1, said override means lost motion meansincluding linkage connecting said pressure responsive means and saidproportioning valve means normally fixed valve element and acting toallow a predetermined initial amount of movement of said pressureresponsive means while permitting proportioning of pressure when thepressures in said first and second brake sub-systems are approximatelynormal, and acting only after said predetermined initial amount ofmovement of said pressure responsive means to an actuating position inresponse to pressure loss in the other of said first and second brakesub-systems to remove said normally fixed valve element to an open valveposition permitting pressure in said one brake system to pass throughsaid proportioning valve means without being proportioned.
 3. Themechanism of claim 1 further comprising metering valve means in serieswith the other of said first and second brake sub-systems with the inputto said metering valve means being in fluid pressure communication withthe other of said first and second piston effective areas.
 4. In a brakesystem for a vehicle having first and second separate brake sub-systems:brake system pressure loss indicating means for providing a signal inresponse to pressure loss in either of said brake sub-systems, saidindicating means comprising pressure responsive means having first andsecond opposed effective areas respectively responsive to the pressuresin said first and second brake sub-systems, and normally being in afirst condition in response to the pressures in both sub-systems beingapproximately normal, and being moved through a predetermined amount ofinitial movement and therebeyond to a second condition in response topressure loss in either but only one sub-system below the approximatelynormal sub-system pressures; and signal means for providing said signalin response to said pressure responsive means being actuated to saidsecond condition; proportioning valve means in series with said secondbrake sub-system with the input to said proportioning valve means beingin fluid pressure communication with said second opposed effective areaof said pressure responsive means, said proportioning valve means havinga first valve element normally having a fixed position and a secondvalve element normally movable relative to said first valve element inresponse to the pressure in said second brake sub-system at the input tosaid proportioning valve means to proportion pressure at the output ofsaid proportioning valve means; and proportioning valve means overridemeans including said first valve element and lost motion connectingmeans received within and inclUding one end of said pressure responsivemeans and connecting said first valve element and said pressureresponsive means for movement of said first valve element in a directionaway from said second valve element when there is a pressure loss onlyin said first brake sub-system which is sufficient to move said pressureresponsive means beyond said predetermined amount of initial movement tooverride the proportioning action of said proportioning valve means andpermit full transmission of second brake sub-system pressure throughsaid proportioning valve means between said valve elements.
 5. In abrake system for a vehicle having first and second separate brakesub-systems: brake system pressure loss indicating means for providing asignal in response to pressure loss in either of said brake sub-systems,said indicating means comprising pressure responsive means having firstand second opposed effective areas respectively responsive to thepressures in said first and second brake sub-systems, and normally beingin a first condition in response to the pressures in both sub-systemsbeing approximately normal, and being actuatable to a second conditionin response to pressure loss in either but only one sub-system below theapproximately normal sub-system pressures, and signal means forproviding said signal in response to said pressure responsive meansbeing actuated to said second condition; and proportioning valve meansin series with one of said first and second brake sub-systems with theinput to said proportioning valve means being in fluid pressurecommunication with one of said first and second opposed effective areas,said proportioning valve means including a housing having a boretherein, an annular seal support member and an annular valve cap membermounted in said housing bore, said members and said housing bore beingsubstantially axially aligned and said members receiving therein apressure responsive movable proportioning valve element in reciprocablypiloted relation, said annular valve cap member having the pilotingsurface thereof extending axially a distance greater than the allowablemovement of said valve element and said annular seal support memberhaving an axially extending piloting surface of said valve elementthereat for bind-free movement of said valve element relative to thepiloting surfaces of said members.
 6. The mechanism of claim 5, saidother annular seal support member piloting surface being coated prior toassembly with a soft metallic plating, of tin, lead or cadmium by way ofexample, to a radial thickness at least equal to the tolerance allowablefor misalignment of said piloting surfaces, said plating beingsufficiently soft to wear with a few strokes of said valve element toestablish a close alignment and prevent valve binding.
 7. The mechanismof claim 6, said soft metallic plating radial thickness being at least0.0002 inch.
 8. The mechanism of claim 6, said soft metallic platingradial thickness being in the range of 0.0002 inch to 0.0004 inch.
 9. Ina brake system for a vehicle having first and second separate hydraulicbrake sub-systems: a proportioning valve assembly in one of said brakesub-systems, said valve assembly having a brake supply pressure inletand a brake apply pressure outlet and a bore connecting said inlet andsaid outlet, a proportioning piston mounted in said bore for limitedreciprocal movement, a first stop member mounted in said bore andlimiting movement of said piston toward said inlet, a second stop membermounted in said bore and limiting movement of said piston toward saidoutlet, a first proportioning valve element formed on said piston, asecond proportioning valve element reciprocably mounted in said bore andhaving a normally fixed position defined by one of said stop members andcooperating with said first proportioning valve element to control fluidflow and pressure transmittal between said inlet and saiD outlet, firstmeans yieldably urging said piston toward and into engagement with saidsecond stop member, second means yieldably urging said secondproportioning valve element toward said first proportioning valveelement, a snubbing member on one end of said piston engaging saidsecond stop member, said piston having opposed differential areas withthe smaller area being exposed to brake supply pressure at said inletand the larger area being exposed to brake apply pressure at said outletto generate a force opposing said first yieldable means and at apredetermined brake supply pressure moving said piston and said firstproportioning valve element into proportioning relation with said secondproportioning valve element to proportion the brake apply pressurerelative to the brake supply pressure, the brake supply pressuredecreasing upon brake release and the brake apply pressure then actingon said piston against said first yieldable means to move said pistonand said proportioning valve elements toward said inlet and initiallydecrease said brake apply pressure by piston displacement until saidpiston engages said first stop member, said second proportioning valveelement being moved by the higher brake apply pressure against thedecreased brake supply pressure and the force of said second yieldablemeans to move away from said first proportioning valve element to permitbrake fluid flow from said outlet to said inlet and further decrease ofbrake apply pressure, said first yieldable means then overcoming theforce generated by the decreasing brake apply pressure acting on saidpiston larger differential area and moving said piston away from saidfirst stop member toward said second stop member, said snubbing memberengaging said second stop member to cushion the engagement of saidpiston with said second stop member; and said second yieldable meansmoving said second proportioning valve element to its normally fixedposition spaced from said first proportioning valve element to maintainfluid communication between said inlet and said outlet during theremainder of the brake release cycle and the initial part of the nextbrake apply cycle.